Virtually all of the fixtures used to light stage shows, concerts, discotheques, films, and television productions produce light beams of directional character, beams whose azimuth and elevation relative to the structure supporting the fixture must be adjusted in order to illuminate the desired subject. In almost every case, the elevation adjustment capability is provided by suspending the fixture housing in the opening of a fork-like yoke or frame. The required azimuth adjustment capability is provided by rotatably mounting the yoke to the supporting structure. The fixture housing is manually rotated in each axis and locked in place by friction at the pivots as disclosed, for example, in U.S. Pat. No. 1,977,883.
This method of adjustment is simple and inexpensive, but it has the disadvantage that the maximum azimuth adjustment on either side of a plane perpendicular to the long axis of a common support, for any given fixture length greater than the fixture width, is inversely related to the spacing between the fixture and an adjacent fixture or obstruction. This requires that the spacing between adjacent elongated fixtures on a common support be increased in order to provide adequate clearance for azimuth adjustment, with the undesirable effect of thus decreasing the number of fixtures which can be accommodated on a given support.
Those fixtures which allow remote adjustment of azimuth and elevation generally employ this method with the addition of motors at the pivots, for example, as disclosed in U.S. Pat. No. 1,680,685.
This method of remote azimuth and elevation adjustment has several important disadvantages. One is that the entire mass of both the fixture and its yoke must be suspended from the azimuth pivot, and a motor and drive system must be provided having sufficient torque to accelerate this mass to motion and then smoothly decelerate it to a stop at the required position with the contradictory objects of both maximum speed and accuracy. The azimuth pivot must further maintain a high degree of stiffness so as not to introduce undesirable motion or error into positioning while minimizing friction, requiring the use of expensive bearings. Further, the very length of the fixture housing increases the problem of inertia presented by its moving mass. Also, changes in the design of the fixture which increase its length, or weight, or change its center-of-gravity can have such impact on the requirements for the azimuth actuator that major changes in the actuator and/or drive may be required. One such change is the addition of a motorized color changer mounted to the front of the fixture. Finally, most productions demand far more azimuth adjustment of a fixture than elevation adjustment, placing the highest workload on the weakest link in the system.
Alternatively, U.S. Pat. Nos. 1,747,279 and 4,112,486 employ an inverted "L" half-yoke. To all of the disadvantages of the more conventional full yoke must be added the complications of the eccentric loading of both the yoke itself and of the azimuth pivot.
Alternatively, some fixtures have been built with the use of a mirror rotatable in two axes as the method of adjusting azimuth and elevation, a remotely adjustable version disclosed in U.S. Pat. No. 2,054,224. While the motorization of such a fixture is far simpler because of the minimal mass of the mirror, this approach sets severe and generally unacceptable limits on adjustability, for while the azimuth adjustment of such a system is unrestricted, elevation adjustment is limited to within a narrow range on either side of a plane perpendicular to the elongated axis of the fixture housing. Adjustment towards the fixture results in obstruction of the beam by the fixture housing itself, while adjustment away from it results in clipping of the beam as it elongates beyond the edges of the mirror.
Alternatively, U.S. Pat. Nos. 4,663,698 and 4,729,071 disclose fixtures employing at least two mirrors for beam azimuth and elevation adjustment. In such systems, the fixture beam is directed downward towards a first mirror which is carried on a first support rotatable relative to the fixture housing about a vertical axis. This first support carries a second support with a mirror which is rotatable relative to the first support about a horizontal axis. The light beam is relayed from mirror to mirror, and the rotation of the two supports provides for adjustment of the beam in two axes.
Such systems have many practical disadvantages. In the case of the system disclosed in U.S. Pat. No. 4,663,698, such disadvantages include the height and width of the mirror system; the offset mounting of the second support, which produces a moment arm requiring greater effort to accelerate and decelerate under positive control; and the eccentric loading of the rotational couplings between the first support and the fixture and between the first support and the second support, which complicates their design and actuation - particularly given the requirements for both low friction and a high degree of stiffness while affording a large and unobstructed opening through the center for passage of the light beam.
The system disclosed in U.S. Pat. No. 4,663,698 adds a third and fourth mirror to bring the mirror responsible for elevation adjustment into alignment with the vertical axis of rotation. The system, however, suffers from many of the same disadvantages, plus those of its lengthened optical path.
Both systems place the highest workload--the pan or azimuth adjustment--on the portion of the system required to move the most mass.
It is the object of the present invention to disclose an improved apparatus for adjusting beam azimuth and elevation having none of the disadvantages of prior art methods.